|BOOT(8)||System Manager's Manual (amd64)||BOOT(8)|
— amd64-specific second-stage bootstrap
The main purpose of this program is to load the system kernel while dealing with the pitfalls of the PC BIOS architecture.
As described in boot_amd64(8), this program is loaded by the biosboot(8) primary bootstrap loader and provides a convenient way to load the kernel. This program acts as an enhanced boot monitor for PC systems, providing a common interface for the kernel to start from.
Basic operations include:
The sequence of its operation is as follows: initialization,
parsing the configuration file, then an interactive command line. While at
the command line you have 5 seconds to type any commands, if needed. If time
expires, the kernel will be loaded according to the current variable
settings (see the
set command). Each time a kernel
load fails, the timeout is increased by one second. The sequence of
boot operations is as follows:
pc0) and up to four serial consoles (
com3) connected to the serial ports. Display messages to the default console about the devices found.
bootwas loaded from, open and parse it. Lines beginning with the ‘#’ character, as well as whitespace at the beginning of lines, are ignored. The file may contain any commands
bootaccepts at the interactive prompt. Though default settings usually suffice, they can be changed here.
boot.conf processing can be skipped,
and the automatic boot cancelled, by holding down either Control key as
>> OpenBSD/amd64 BOOT [x.xx]
is displayed to the active console, where
x.xx is the version number of the
boot program, followed by the
prompt, which means you are in interactive mode and may enter
commands. If you do not,
boot will proceed to
load the kernel with the current parameters after the timeout period has
boot attempts to load the
kernel executable /bsd. If it fails to find the
kernel and no alternative kernel image has been specified, the system will
be unable to boot.
boot supports booting from
softraid(4) RAID 1 and
The following commands are accepted at the
bootvariables will be used.
When selecting the device to boot from,
boot makes no distinction between SCSI and IDE
type drives; they are detected as ‘hd’ devices. Therefore,
to boot kernel /bsd from slice ‘a’
on the first hard drive (irrespective of device type), specify
machine boot hd0a
Where a is the first MBR partition table entry, and d the last.
Meaning to add(+), exempt(-) or limit(=) the amount of memory specified by size at the location specified by address. Both size and base address can be specified as octal, decimal, or hexadecimal numbers, as accepted by the strtoul(3) routine. Memory size may be suffixed by K or k, for kilobytes; M or m, for megabytes; and G or g, for gigabytes.
The limit(=) option simply ignores any memory above the given memory limit. This is useful for testing kernels in an artificially constrained memory situation. For example, the following limits the kernel to using only memory below 64MB:
machine mem =64M
Memory segments are not required to be adjacent to each other; the only requirement is that there is real physical memory under the range added. The following example adds 32MB of memory right after the first 16MB:
machine mem +0x2000000@0x1000000
Another useful command is to withdraw a range of memory from OS usage (it may have been wrongfully reported as useful by the BIOS). This example effectively excludes the 15–16MB range from the map of useful memory:
machine mem -0x100000@0xf00000
bootwas compiled with DEBUG defined.
bootoffers you five seconds of grace time before committing the change to allow you to change your terminal's speed to match. If changing speed not for the active console, the baudrate is set for the next time you switch to a serial console. The baudrate value is not used for the
The default baudrate is 9600bps.
Boot the default kernel:
Remove the 5 second pause at boot-time permanently, causing
boot to load the kernel immediately without
# echo "boot" > /etc/boot.conf
Use serial console. A null modem cable should connect the specified serial port to a terminal. Useful for debugging.
boot> set tty com0
Invoke the serial console at every boot:
# echo "set tty com0" > /etc/boot.conf
Boot the kernel named /bsd from the second
hard disk in “User Kernel Configuration” mode (see
mechanism allows for the explicit enabling and disabling of devices during
the current boot sequence, as well as the modification of device parameters.
Once booted, such changes can be made permanent by using
boot> boot hd1a:/bsd -c
This program was written by Michael Shalayeff for OpenBSD 2.1 on the i386 platform, and was later ported to the amd64 platform.
|December 23, 2019||OpenBSD-6.7|